1. Field of the Invention
The present invention relates to an intake system of an engine.
2. Description of the Prior Art
Generally, the intake gas introduced in the engine becomes less ignitable and Combustive in a lower engine load condition where a smaller amount of the intake gas is introduced into the engine in addition, so-called lean burn engine making the intake gas leaner as the engine load is reduced has been prevailing. In such lean burn engine, the combustibility of the intake gas is further deteriorated.
In view of this, it has been proposed an engine having a primary intake port offset from the center of the cylinder bore and introducing the intake gas throughout entire engine load condition and a secondary intake port opened in intermediate and high engine load conditions.
The intake gas is introduced through only the primary port in the low engine load condition and through both the primary and secondary ports in the intermediate and high engine load condition. These type engines are disclosed in, for example, Japanese Patent Public Disclosure Nos. 59-54732, laid open to the public in 1984, 61-218726, laid open to the public in 1986, Japanese Utility Model Public Disclosure Nos. 61-175541, laid open to the public in 1986 and 61-84135, laid open to the public in 1986. In this type of engine having the primary and secondary ports, intake gas is introduced into the combustion chamber of the engine from the primary port forming a swirl current flowing along circumferential wall of the cylinder when the engine load is relatively low and the secondary port is closed. Thus, this swirl facilitates forming dense intake gas layer around the ignition plug so that the ignition and combustion characteristics of the intake gas are enhanced. In the intermediate and high engine load conditions, the intake gas are introduced into the combustion chamber through both the primary and secondary ports so that the charging efficiency of the intake gas is improved to provide an increased engine output.
In such an engine having both the primary and secondary ports, there occur some situations where it is preferred that the swirl flow is formed in the combustion chamber when both the primary and secondary ports are opened. In the conventional engine, the intake gas flow through the secondary port forms a swirl flow against that of the intake gas through the primary port. In other words, the intake gas from the secondary port flows in the reverse direction of the intake gas from the primary port along the circumferential wall of the cylinder to weaken the swirl flow from the primary port. Therefore, it is difficult to improve the ignition and combustion characteristics once the secondary port is opened to introduce a part of the intake gas. In the engine disclosed in the Japanese Utility Model Public Disclosure No. 61-175541, it is proposed that the primary and secondary ports are oriented in the substantially same direction to reduce the reverse flow from the secondary port against the swirl flow of the primary port. However, this structure is not good enough to produce a desirable swirl flow of the intake gas in the combustion chamber.